Davis A P, Capecchi M R
Howard Hughes Medical Institute, Department of Human Genetics, University of Utah School of Medicine, Salt Lake City 84112, USA.
Development. 1996 Apr;122(4):1175-85. doi: 10.1242/dev.122.4.1175.
Using gene targeting in mice, we have undertaken a systematic mutational analysis of the homeobox-containing 5' HoxD genes. In particular, we have characterized the limb defects observed in mice with independent targeted disruptions of hoxd-12 and hoxd-13. Animals defective for hoxd-12 are viable, fertile, and appear outwardly normal yet have minor autopodal defects in the forelimb which include a reduction in the bone length of metacarpals and phalanges, and a malformation of the distal carpal bone d4. The limb phenotypes observed in hoxd-13 mutant mice are more extensive, including strong reductions in length, complete absences, or improper segmentations of many metacarpal and phalangeal bones. Additionally, the d4 carpal bone is not properly formed and often produces an extra rudimentary digit. To examine the genetic interactions between the 5' HoxD genes, we bred these mutant strains with each other and with our previously characterized hoxd-11 mouse to produce a series of trans-heterozygotes. Skeletal analyses of these mice reveal that these genes interact in the formation of the vertebrate limb, since the trans-heterozygotes display phenotypes not present in the individual heterozygotes, including more severe carpal, metacarpal and phalangeal defects. Some of these phenotypes appear to be accounted for by a delay in the ossification events in the autopod, which lead to either the failure of fusion or the elimination of cartilaginous elements. Characteristically, these mutations lead to the overall truncation of digits II and V on the forelimb. Additionally, some trans-animals show the growth of an extra postaxial digit VI, which is composed of a bony element resembling a phalange. The results demonstrate that these genes interact in the formation of the limb. In addition to the previously characterized paralogous interactions, a multitude of interactions between Hox genes is used to finely sculpt the forelimb. The 5' Hox genes could therefore act as a major permissive genetic milieu that has been exploited by evolutionary adaptation to form the tetrapod limbs.
利用小鼠基因打靶技术，我们对含同源异型框的5’HoxD基因进行了系统的突变分析。特别是，我们对hoxd - 12和hoxd - 13独立靶向破坏的小鼠中观察到的肢体缺陷进行了特征描述。hoxd - 12缺陷的动物能够存活、繁殖，外表看起来正常，但前肢有轻微的足部缺陷，包括掌骨和指骨长度缩短，以及远侧腕骨d4畸形。在hoxd - 13突变小鼠中观察到的肢体表型更为广泛，包括许多掌骨和指骨长度大幅缩短、完全缺失或节段异常。此外，d4腕骨形成不当，常常长出一个额外的残指。为了研究5’HoxD基因之间的遗传相互作用，我们将这些突变株相互杂交，并与我们之前鉴定的hoxd - 11小鼠杂交，产生了一系列反式杂合子。对这些小鼠的骨骼分析表明，这些基因在脊椎动物肢体形成过程中相互作用，因为反式杂合子表现出单个杂合子中不存在的表型，包括更严重的腕骨、掌骨和指骨缺陷。其中一些表型似乎是由于足部骨化事件延迟导致的，这会导致融合失败或软骨元件消失。典型的是，这些突变导致前肢的II和V指整体截断。此外，一些反式动物长出了一个额外的轴后VI指，它由一个类似指骨的骨元件组成。结果表明，这些基因在肢体形成过程中相互作用。除了之前鉴定的旁系同源相互作用外，Hox基因之间的多种相互作用被用于精细塑造前肢。因此，5’Hox基因可能作为一个主要的许可遗传环境，在进化适应过程中被利用来形成四足动物的肢体。
